xen/blkfront: realloc ring info in blkif_resume

Need to reallocate ring info in the resume path, because info->rinfo was freed
in blkif_free(). And 'multi-queue-max-queues' backend reports may have been
changed.

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Reported-and-Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>